During the past twenty years, the developments in the field of manufacturing very large scale integrated circuits have been phenomenal. Circuits that used to occupy an entire room have been shrunken into a small integrated circuit, which may fit into, for example, a small calculator or computer. And still the aim of many in the industry is to further reduce the size of circuits so as to occupy even smaller areas.
As circuits are reduced in size, each component within each circuit is likewise reduced in size. The problem, however, lies in manufacturing these smaller components without degrading the function and performance of the circuits. This is where each component's line and space definitions are crucial in the production of these integrated circuits. Such line and spacing definitions are approaching dimensions near a tenth of a micron. Therefore, there is a need to have accurate equipment and manufacturing techniques in manufacturing these types of integrated circuits.
One equipment commonly used in manufacturing very large scale integrated circuit is a photostepper. A photostepper is used to expose a layer of resist disposed over a wafer to electromagnetic radiation spatially modulated with a circuit pattern. The photostepper usually steps and repeats the exposure of the resist so as to form images of multiple circuit patterns on the resist. The wafer is subsequently removed and subjected to an etching process so as to leave a pattern disposed on the wafer defining the multiple circuit patterns.
The manner in which a photostepper exposes the layer of resist is by projecting an image of the circuit pattern towards the wafer. A reticle or mask having a series of darken images disposed thereon defining the circuit pattern is interposed between a light source and the wafer, and a controllable shutter is interposed between the mask and the wafer. The light source is constantly energized and the photostepper periodically opens its shutter so that electromagnetic energy having the proper wavelength emanating from the light source and propagating through the image on the mask strikes the resist. The shutter is thereafter moved to a different position over the wafer and another exposure is performed. Generally, this process is repeated until an array of exposures is formed on the resist.
When exposing the circuit pattern images onto the resist, it is important that the image be properly focused. Otherwise, blurred images of the circuit pattern will form on the resist resulting in blurred circuit patterns when the resist in subsequently developed. Therefore, the operator of the photostepper must insure that the images of the circuit patterns are optimally focused onto the resist.
Some photosteppers relieve the operator of this duty by providing an automatic focus sensor. An example of such a photostepper is the Canon model no. 2500i3, and whose operation manual, entitled "FPA-2500 i2/i3" and published in 1992, is herein incorporated by reference. The automatic focus sensor scans the surface of the wafer and determines an optimum focus setting for the region that was scanned. Once this is performed, the photostepper then proceeds to expose the resist using the optimum focus setting.
The developing of a circuit on a wafer may comprise many processing steps including depositing and etching of thin-films and the wafer itself. Because of prior multiple depositing and etching processes, a wafer may obtain a surface with large varying topography heights. The topography height of a wafer is the height of a particular feature on the surface of the wafer relative to a reference height on that surface. Some features will have low topography heights because it has undergone, for example, multiple etching steps. Other features will have high topography heights because it has undergone, for example, multiple depositing steps. This presents a problem for the automatic focus sensor. Because the focus sensor finds the optimum focus for a particular topography height, the optimum focus will not correspond to all surfaces in a region having substantial variation in its topography height.
Some photosteppers resolve this problem by providing an average topography height function. An example of such a photostepper is again the Canon model no. 2500i3. The average topography height function works in conjunction with the automatic focus sensor of the photostepper. The automatic focus sensor scans and records the optimum focus for a multitude of topography heights across a region on the surface of the wafer. The average topography height function takes these focus readings and calculates an average topography height for the region and an optimum focus setting for that height. Once this occurs, the photostepper is ready to expose the layer of resist over that region using the average topography height's optimum focus setting. If, however, the average topography height function is not properly obtaining the average topography height, the photostepper will be exposing the resist over that region with a focus setting that is not optimum.
Another concern is whether the average topography height function can track the average topography height of various regions across the wafer having correspondingly different average topography heights. This concern arises, for example, if the various regions on the surface of the wafer have been reserved for different circuit patterns. Obviously, different circuit patterns are going to have different topographies, and accordingly, different average topography heights. During a production run, the photostepper must change its focus setting so that each region may be exposed using its optimum focus setting. Therefore, it is important that the average topography height function can track the various average topography height of various regions across the wafer.
Yet another matter of concern is whether the average topography height function can track the average topography height of various regions across the wafer having correspondingly different etch densities. The etch density of a region is defined as the area of the region on the surface of the wafer that have been etched divided by the total area of the region. An etch density of 100 percent, for example, means that the region has been totally etched. Whereas an etch density of 50 percent means that half the area of the region has been etched and the other half has not. The etch density of a wafer may vary if, for example, various regions on the wafer are reserved for the production of different circuits. Therefore, it is important that the average topography height function of the photostepper track the average topography height of various regions across the wafer having correspondingly different etch densities.